This invention generally relates to the field of injection molding presses, and more particularly relates to rotary injection molding presses which inject fluid plastic into a series of individual mold blocks positioned in a circle on a rotatable platform.
Injection molding is typically used for molding small plastic articles, and is often used where a component, such as an electrical connector, is molded therein. In a rotary injection molding press, a series of mold blocks are mounted in a circle on a rotatable platform or table. An injection mechanism is fed with solid plastic, usually in pellet form. The injection mechanism includes heating or cooling elements for heating or cooling the plastic into a semi-fluid state, and a reciprocating screw element to force the semi-fluid plastic under pressure through an injector nozzle into the cavity of a properly-positioned mold block. Each of the mold blocks is typically constructed to have a bottom or base portion secured to the rotatable platform, and a hinged top portion which can be opened to remove the molded plastic article from the molding cavity. Heat is provided during the molding operation by electrical resistance heating.
During the first step of the injection molding operation, an opened mold block is positioned into a preparation station, wherein the electrical connectors or other components are inserted into the heated mold block and the mold block is closed. Next, the closed mold block is positioned into the injection station, wherein the reciprocating screw is engaged and the mold block is injected with semi-fluid plastic. After a particular mold block has been injected, the rotatable platform is indexed to position the next block under the injection mechanism. As the platform is rotated in this indexing fashion, the previous mold block is positioned into a curing station, wherein the fluid plastic within the mold cavity is allowed to heat and harden to form the desired plastic article. Next, the platform is indexed to an eject station, wherein the molded plastic article is removed from the mold block. This indexed rotation of the mold blocks on the rotatable platform continues, thus resembling a circular assembly line for molding plastic articles.
One of the more common problems associated with rotary injection molding is that of how to control the temperature of the mold blocks, particularly if thermoset plastic is used. If the temperature of the mold does not reach the required temperature for properly curing a thermoset plastic, the plastic article will crumble. On the other hand, if the temperature of the mold rises above the recommended maximum temperature for curing the particular thermoset plastic used, then the plastic will burn and the article will be discolored and/or weakened. More specifically, if a type SE 851/852 or SE 871 silicone rubber thermoset compound manufactured by General Electric under the trade name TUFEL.RTM. is used for injection molding of small articles, the manufacturer recommends that the temperature tolerance of the molds for a one-minute cure time be .+-.3.degree. C. for proper curing. Hence, accurate temperature control must be maintained within the molding blocks throughout the molding operation.
Although independent control of the temperature of each of the mold blocks individually is not necessarily a requirement for many applications, independent temperature control of each of the mold blocks provides for a higher level of quality control over the operation. Individual temperature control also permits the molding of different articles requiring different molding temperatures on the same rotatable platform.
In order to provide individual temperature control of each mold, some type of temperature monitoring device must be affixed to each of the individual mold blocks. Either thermocouples, each having two wire leads, or resistance temperature devices (RTDs), each having two or three wire leads, must be connected to the temperature control unit. For example, if there are six mold blocks on the rotatable platform, then at least a dozen wire leads from the temperature monitoring devices must be routed to the temperature control unit. Furthermore, the outputs of the temperature control unit must also be connected to the individual mold blocks to independently control the temperatures. That is, the individual electric heater cartridges, each having two wire leads, also require connection to the temperature control unit. Thus, the temperature control unit would require at least twenty-four individual wire leads for a rotatable platform having six mold blocks.
Typically, the temperature control unit is located next to the injection mechanism on the stationary frame of the injection molding press. If this configuration is used, some type of rotary electrical coupler must be used to supply electrical connection to the heater cartridges and the thermocouples. In the past, brushes and slip-rings have been used to provide these electrical connections. For example, the slip rings would surround a post extending upwardly along the axis of the rotatable platform, such that a number of brushes would contact each slip-ring.
There are several drawbacks inherent in this approach. First, brushes and slip-rings are subject to wear and oxidation, such that electrical intermittence often occurs. If the intermittence is located in the feedback control loop of the temperature control unit, i.e., in series with the thermocouple, then accurate temperature control is very difficult to maintain. Similarly, if the intermittence is located in the high-current heater circuit, then arcing and burning of the contacts inherently occurs, thus reducing the life of the contacts and increasing their resistance. Numerous safety issues are also involved if the high voltage contacts are left out in the open. Still further, the use of numerous slip-ring contacts, i.e., twenty-four contacts in the previous example, is very expensive and complex to construct, and even more costly and difficult to maintain.
A need, therefore, exists for a reliable and cost effective way to provide individual temperature control for the individual mold blocks of a rotary injection molding press.